Inkjet recording is a recording technique that performs recording of an image, characters or the like by causing tiny droplets of ink to fly in accordance with one of various operation principles and then allowing them to adhere on a recording medium such as paper. Inkjet recording features high-speed printing performance, low operating noise, applicability for the recording of a wide variety of characters and patterns, easy multicolor printing, and obviation of development and image fixing. In particular, an image formed by multicolor inkjet recording can provide a record which is no way inferior to an image printed by multicolor printing making use of a form-plate-dependent printing technique or by a color photographic technique. Multicolor inkjet recording has a still further merit in that, when the number of copies or prints to be made is small, it requires lower printing cost than an ordinary printing technique or photographic technique. Multicolor inkjet recording is, therefore, rapidly finding wide-spread utility as image recorders for various information equipment in recent years. For example, multicolor inkjet recording is finding increasing utility in a wide variety of fields in which recording of full-color images is required, for example, output of image designs in design business, production of color block copies in a printing field where the quality of photographic images is required, and production of billboards and catalogs which are frequently updated.
Further, owing to the spreading of digital still cameras and computers, there are greater opportunities to output photographic images simply by inkjet printers. From such a background, images recorded by inkjet printers are required to have high resolution and vividness equivalent to silver halide pictures, leading to demands for recording apparatuses, recording media and inks of higher performance.
As inks loaded on inkjet printers these days, those using as colorants water-soluble dyes excellent in color developing ability constitute the mainstream. However, a water-soluble dye has high hydrophilicity so that, when an image is stored under high humidity for a long time or water drops adhere a recorded surface subsequent to recording of the image with such an ink, the dye with which the image is formed tends to bleed. Further, use of a water-soluble dye ink, especially on a recording medium having a porous ink-absorbing layer makes it possible to obtain an unblurred uniform image, but involves a problem that subsequent to the recording, the image is prone to fading.
To overcome these problems, water-based dispersion inks are finding utility. Each of these water-based dispersion inks contains a water-insoluble colorant, such as a pigment, dispersed in water by using a hydrophilic high-molecular compound and a surfactant. Images formed with these inks are excellent in light fastness, waterproofness and ozone resistance. For high image fastness, these inks are therefore attracting interests as replacements for dye inks.
Compared with dye inks, however, water-based dispersion inks with pigments contained therein are lower in tinting efficiency so that they are required in greater amounts to record images. This means that these water-based dispersion inks require recording media excellent in ink absorbency. With a recording medium with an ink-receiving layer simply rendered thick to provide improved ink absorbency, however, the colorant penetrates deep into the ink-receiving layer so that the colorant exhibits inferior color-developing ability. With a recording medium prepared by placing importance on the color-developing ability of a colorant, on the other hand, (an ink solvent runs over on the ink-receiving layer, thereby developing a problem such as image bleeding.
When an image is formed on a recording medium, the surface of which has been smoothened to impart gloss, by using a water-based dispersion ink with a pigment contained as a colorant therein, particles of the pigment cannot enter an ink-receiving layer so that the pigment particles remain on the surface. The image is, therefore, provided with lowered abrasion resistance. An attempt has, therefore, been made to provide the surface of an ink-receiving layer with some asperity. With this method, however, no satisfactory results have been obtained yet with respect to the ink absorbency upon forming an image or the surface gloss of the image so obtained.
As other methods for providing an ink-receiving layer with improved ink-fixing property, many methods have also been disclosed, each of which features incorporation of a cationic polymer having tertiary or quaternary ammonium salts in the ink-receiving layer. To improve the ink-fixing property of an ink-receiving layer, however, it is necessary to incorporate a predetermined or greater amount of such a cationic polymer in the ink-receiving layer. This leads to an increase in the amount of the cationic polymer in the ink-receiving layer. Accordingly, the content of fine inorganic particles in the ink-receiving layer is relatively decreased, thereby making it difficult to retain high ink-absorbing ability. In addition, the cationic polymer has a potential problem in that it may deteriorate the stability of a coating formulation required for the formation of the ink-receiving layer. This tendency becomes more pronounced as the content of the cationic polymer increases. The incorporation of such a cationic polymer, therefore, is accompanied by problems that the coating stability of the coating formulation is significantly reduced and the ink-receiving layer formed subsequent to the coating is provided with deteriorated surface smoothness and hence, with lowered gloss.
Further methods have, therefore, been proposed as new techniques. According to these methods, two solutions are used, one being an ink and the other an auxiliary solution for forming agglomerates when brought into contact with the ink, to improve the fixing property and waterproofness of a colorant on a recording medium. Before injecting the ink, the auxiliary solution is caused to adhere on the recording medium such that its ink-fixing property is improved.
For example, JP 63-29971 A discloses to cause a solution, which contains an organic compound having two or more cationic groups per molecule, to adhere on a recording medium and then to perform recording on the recording medium with an ink containing an anionic dye. JP 64-9279 A discloses to cause an acidic solution, which contains succinic acid or the like, to adhere on an ink-receiving layer and then to perform recording on the ink-receiving layer with an ink containing an anionic dye. JP64-63185 A discloses, specifically to cause a dye-insolubilizing solution to adhere before recording with an ink. Further, JP2002-302627 A discloses to cause a reaction mixture, which contains an anionic reactant and an anionic resin emulsion, to adhere on a recording medium and then to cause a water-based ink composition, which contains a colorant and a cationic resin emulsion, to adhere on the recording medium.
Even in the above-described methods, a recording medium the ink absorbency of which is low causes thickening of an ink only on the surface of the recording medium, thereby inducing an uneven distribution of the ink on the recording medium and a reduction in image uniformity. Further, the image-forming surface is hard to dry so that an image formed thereon is provided with lowered abrasion resistance. These methods are, therefore, accompanied by a drawback that, when images are formed, for example, on sheet-form recording media by continuous printing and the recorded matters are stacked one over the other, the recorded matters are smeared by offsetting or the like of the ink because the ink-absorbing property is insufficient. In addition, these methods are disadvantageous in cost because each printer requires an ejection system exclusively for the above-described auxiliary solution.